This invention relates to microwave oscillators, and more particularly to oscillators which include a Gunn diode as an active element in a resonator circuit. Although it is possible to implement a microwave oscillator of this kind in a waveguide structure, it has proved difficult to produce a satisfactory oscillator in an integrated form on a planar circuit substrate. The planar integrated form is preferred in principle since the techniques which can be used to fabricate it are inherently more reproducable, leading to more controllable and predictable circuits. However, microwave oscillators produced in planar integrated form have not exhibited sufficiently good frequency or power characteristics.